feat: allow passing initial value to fiber function parameter on first call

When a fiber is started for the first time via call() or transfer(), the value argument is now bound to the fiber's function parameter if it takes one. Previously, the first value was always ignored. Also adds runtime error for fibers created with functions taking more than one parameter, and updates documentation and tests accordingly.

test/core/fiber/call.wren

@@ -3,5 +3,5 @@ var fiber = Fiber.new {
 }
 
 System.print("before") // expect: before
-fiber.call()       // expect: fiber
+fiber.call()           // expect: fiber
 System.print("after")  // expect: after

test/core/fiber/call_to_parameter.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.new {|value|
+  System.print(value)
+}
+
+System.print("before") // expect: before
+fiber.call()           // expect: null
+System.print("after")  // expect: after

test/core/fiber/call_with_value.wren

@@ -2,8 +2,6 @@ var fiber = Fiber.new {
   System.print("fiber")
 }
 
-// The first value passed to the fiber is ignored, since there's no yield call
-// to return it.
-System.print("before")    // expect: before
-fiber.call("ignored") // expect: fiber
-System.print("after")     // expect: after
+System.print("before") // expect: before
+fiber.call("value")    // expect: fiber
+System.print("after")  // expect: after

test/core/fiber/call_with_value_to_parameter.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.new {|value|
+  System.print(value)
+}
+
+System.print("before") // expect: before
+fiber.call("value")    // expect: value
+System.print("after")  // expect: after

test/core/fiber/new_wrong_arity.wren

@@ -0,0 +1 @@
+var fiber = Fiber.new {|a, b| null } // expect runtime error: Function cannot take more than one parameter.

test/core/fiber/transfer_to_parameter.wren

@@ -0,0 +1,24 @@
+var a = Fiber.new {|param|
+  System.print("a %(param)")
+}
+
+var b = Fiber.new {|param|
+  System.print("b before %(param)")
+  a.transfer()
+  System.print("b after")
+}
+
+var c = Fiber.new {|param|
+  System.print("c before %(param)")
+  b.transfer()
+  System.print("c after")
+}
+
+System.print("start") // expect: start
+
+c.transfer()
+// expect: c before null
+// expect: b before null
+// expect: a null
+
+// Nothing else gets run since the interpreter stops after a completes.

test/core/fiber/transfer_with_value_to_parameter.wren

@@ -0,0 +1,24 @@
+var a = Fiber.new {|param|
+  System.print("a %(param)")
+}
+
+var b = Fiber.new {|param|
+  System.print("b before %(param)")
+  a.transfer("from b")
+  System.print("b after")
+}
+
+var c = Fiber.new {|param|
+  System.print("c before %(param)")
+  b.transfer("from c")
+  System.print("c after")
+}
+
+System.print("start") // expect: start
+
+c.transfer("from main")
+// expect: c before from main
+// expect: b before from c
+// expect: a from b
+
+// Nothing else gets run since the interpreter stops after a completes.

test/core/fiber/yield_with_value.wren

@@ -7,8 +7,8 @@ var fiber = Fiber.new {
 }
 
 System.print(fiber.call())  // expect: fiber 1
-                        // expect: yield 1
+                            // expect: yield 1
 System.print(fiber.call())  // expect: fiber 2
-                        // expect: yield 2
+                            // expect: yield 2
 System.print(fiber.call())  // expect: fiber 3
-                        // expect: null
+                            // expect: null